ICMCTF2013 Session B4-3: Properties and Characterization of Hard Coatings and Surfaces

A novel hollow cathode plasma immersion ion processing method is utilized to deposit diamond like carbon (DLC) coatings on the internal and select external surfaces of metallic pipes and components. The hollow cathode effect (HCE) occurs when high energy electrons oscillate between opposing cathodes causing multiple ionization events. Plasmas taking advantage of the HCE typically have ion and electron densities hundreds of times higher than conventional plasma discharges. This phenomenon allows for very high deposition rates in excess of 3 microns per minute for optimized DLC coating applications as compared to typical chamber based deposition rates on the order of 1-2 microns per hour. DLC coatings have high density, high hardness, and strong adhesion while providing excellent wear resistance with low friction. DLC coatings are also chemically inert in most environments providing protection of the coated substrate from corrosive attack. By introducing different precursor gases, the vacuum-based plasma deposition process can produce coatings with properties tailored for specific applications. In summary, this high speed hollow cathode plasma deposition technology enables DLC based coatings to increase component life in applications where the internal surface of pipes and other parts are exposed to corrosive and abrasive environments.

Fig. 1 : Definition of the coating endurance criterion related to a threshold friction condition.

Fig. 2 : Identification of the DLC coating endurance in the low pressure domain (p_max < 650 Mpa)

Mechanical stresses (extrinsic or intrinsic) are a crucial feature of thin films. The mechanical behaviour of thin film-substrate combinations are strongly affected by internal stresses, especially with respect to the adhesion, durability and tribological performance. Thus, gaining a better understanding of stress-inducing mechanisms is an important concern for surface engineering. Interfacial stress components due to lattice mismatch or different thermal expansion coefficients contribute to the overall film stress as well as dislocations, impurities, voids and grain boundaries. To gain closer insight into the mechanisms of stress development and relaxation, real-time measurements during film growth are necessary. Furthermore, it is of interest to correlate the stresses to the chemical composition and the corresponding microstructure. This is possible in composition spread type materials libraries, where thin films of different compositions are fabricated simultaneously. In order to understand the stress development during sputtering and annealing as a function of composition, high-throughput measurement methods are needed which are both time-resolved and space-resolved.

(Al_100-xCr_x)N thin film materials libraries were fabricated on micro-machined cantilever arrays, in order to simultaneously investigate the evolution of stresses during film growth as well as during thermal processing by analyzing the changes in cantilever curvature. The issue of the dependence of stress in the growing films on composition, at comparable film thicknesses, was investigated. Among the various experimental parameters studied, it was found that the applied substrate bias has the strongest influence on stress evolution and microstructure formation. The compositions of the films, as well as the applied substrate bias, have a pronounced effect on the lattice parameter and the coherence length. For example, applying a substrate bias in general leads to compressive residual stress, increases the lattice parameter and decreases the coherence length. Moreover, bias can change the film texture from [111] orientation to [200]. Further detailed analysis using X-ray diffraction and transmission electron microscopy clearly revealed the presence of a [111] highly textured fcc (B1 type) Al-Cr-N phase in the as deposited state as well as the coexistence of the hexagonal [110] textured Cr₂N phase, which forms in the Cr-rich region. These results show that the combinatorial approach provides insight into how stresses and compositions are related to phases and microstructures of different Al-Cr-N compositions fabricated in the form of materials libraries.

In recent years, the application of AlCrN-type coatings in processes which involve attrition, chipping and/or cracking due to the impacts, such as the industrial metal forming has increased [1]. The cutting edge of coated tools may exceed 1000°C, therefore the oxidation resistance is a very important issue [2]. The oxidation resistance and high temperature mechanical properties of AlCrN are improved comparing with other ternary nitride such as AlTiN [3]. Furthermore, it presents good tribological properties up to 500°C, since a low friction coefficient is generally found even at high temperatures [4].

In this study, AlCrN coatings were prepared by vacuum cathodic arc deposition (CAD). This technique has been chosen due to its versatility, allowing an easy industrial up-scaling.

The correlation of processing parameters was focused on the influence of bias voltage on the resulting mechanical-tribological properties. Indeed, it was demonstrated that the variation of bias voltage by means of the ion peening effect has an important role in modifying the morphological properties - such as surface roughness, crystallite sizes and the associated residual stresses which influences the final properties.

Interesting mechanical properties such as hardness were measured. These properties were strongly affected by the combination of the grain size (Hall-Petch effect) and the intrinsic residual stress.

The tribological behaviour was a consequence of the resulting properties, especially ruled by the H/E ratio.

References

[1] G.S. Fox-Rabinovich et al. Surf. Coat. Technol. 200 (2006) 5738.

[2] H.O. Gekonde et al. Surf. Coat. Technol. 149 (2002) 151.

[3] O. Banakh et al. Surf. Cat. Technol. 163-164 (2003) 57.

[4] R. Franz et al. Tribol. Lett. 23 (2006) 101.